Engines may be configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. Therein, a portion of an engine's cylinders may be disabled during selected conditions defined by parameters such as a speed/load window, as well as various other operating conditions including vehicle speed. An engine control system may disable a selected group of cylinders, such as a bank of cylinders, through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves, and through the control of a plurality of selectively deactivatable fuel injectors that affect cylinder fueling.
Further improvements in fuel economy can be achieved in engines configured to vary the effective displacement of the engine by skipping the delivery of air and fuel to certain cylinders in an indexed cylinder firing pattern, also referred to as a “skip-fire” pattern. One example of a skip-fire engine is shown by Tripathi et al. in U.S. Pat. No. 8,651,091. Therein, an engine controller may continuously rotate which particular cylinders receive air and fuel, which cylinders are skipped, and how many cylinders events the pattern is continued for. By skipping air and fuel deliver to selected cylinders, the active cylinders can be operated near their optimum efficiency, increasing the overall operating efficiency of the engine. By varying the identity and number of cylinders skipped, a large range of engine displacement options may be possible.
However the inventors herein have identified a potential issue with such engine systems. In engine systems that are also configured with variable cam timing (VCT) devices for varying the valve timing of individual cylinders, deactivation of selected cylinders may result in camshaft torque pulses that degrade camshaft phasing. As such, VCT devices may include a vane type cam phaser that is cam torque actuated wherein the actuation of the phaser is dependent on torque generated during cam actuation. Specifically, the torque that the camshaft imparts on the camshaft phaser at any engine angle is determined as a function of the torques imparted on the camshaft from each valve-train member in contact with the camshaft. In engines with the ability to deactivate any number of cylinders, the camshaft torsional signature may vary depending on which cylinders are actively opening and closing valves on a given engine cycle. Since the camshaft phaser depends on the torsional input as the energy source for advancing or retarding the cam phasing, the phaser's ability to phase could be diminished if the torsional signature resulting from the selected cylinder deactivation pattern contains fewer peaks per cycle, or lower amplitude peaks. The inventors have recognized that for a given number of deactivated cylinders, there may be some sets of deactivated cylinders that are able to support engine load requirements, but may not be able to provide sufficient torsional loads capable of phasing the camshafts. As a result, the camshaft phasing ability of the engine may be reduced, degrading engine performance.
In one example, the above issue may be at least partly addressed by a method for an engine comprising: deactivating an individual cylinder valve mechanism according to a cylinder pattern; and reactivating the valve mechanism in response to a request for cam phasing. In this way, cylinder deactivation patterns may be adjusted in the presence of a phasing requirement to allow for improved camshaft phasing.
For example, in response to a drop in engine load, an engine controller may select a cylinder deactivation pattern. The pattern may include a total number of individual cylinder valve mechanisms to be deactivated relative to a total number of active cylinders. The number and identity of cylinders selected for deactivation may be based on the change (that is, drop) in engine load. The individual cylinder valve mechanisms may then be deactivated according to the determined pattern. In response to a cam phasing request, that is a request for advancing or retarding cam timing, the engine controller may reactivate one or more of the deactivated cylinders. In addition, the controller may deactivate one or more of the active cylinders so as to maintain the total number of deactivated/active cylinders constant while adjusting the cylinder deactivation pattern. For example, the controller may compare the camshaft torsion signatures of the different cylinder patterns to identify a pattern with a set of deactivated cylinders that is more favorable for camshaft phasing. As such, for a given total number of deactivated cylinders, there may be a combination of cylinders (based on their identity, firing order, location on engine block, etc.) having a cam torque signature that is more favorable for cam phasing while other combinations are less favorable. For example, the amplitude and position of torsion peaks for each (active) cylinder in an unfavorable cylinder pattern may result in less net energy to phase the camshaft. In comparison, the amplitude and position of torsion peaks for each (active) cylinder in a favorable cylinder pattern may result in more net energy to phase the camshaft. The controller may reactivate deactivated cylinders and/or deactivate active cylinders of the originally selected cylinder pattern to provide the cylinder deactivation pattern that is more conducive for camshaft phasing. Once the camshaft phasing is completed, the controller may adjust cylinder valve deactivation mechanisms to resume the original cylinder pattern (or an alternate pattern based on the current engine load).
In this way, camshaft phasing efficiency may be improved while operating with one or more deactivated engine cylinders. By deactivating and/or reactivating engine cylinders while taking into account the torques imparted on the camshaft from each valve of the engine cylinders, the total energy delivered to the camshaft can be improved. By providing improved torsional load from cylinder valves onto the camshaft while deactivating selected individual cylinder valve mechanisms, the energy for phasing a camshaft is improved. By improving camshaft phasing, cam timing adjustments are better enabled, improving engine performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.